Conventional ultrasound imaging systems comprise an array of ultrasonic transducers which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, the array typically has a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focussed at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focussed reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer.
FIG. 1 depicts an ultrasound imaging system consisting of four main subsystems: a beamformer 2, processors 4 (including a separate processor for each different mode), a scan converter/display controller 6 and a kernel 8. System control is centered in the kernel, which accepts operator inputs through an operator interface 10 and in turn controls the various subsystems. The master controller 12 performs system level control functions. It accepts inputs from the operator via the operator interface 10 as well as system status changes (e.g., mode changes) and makes appropriate system changes either directly or via the scan controller. The system control bus 14 provides the interface from the master controller to the subsystems. The scan control sequencer 16 provides real-time (acoustic vector rate) control inputs to the beamformer 2, system timing generator 24, processors 4 and scan converter 6. The scan control sequencer 16 is programmed by the host with the vector sequences and synchronization options for acoustic frame acquisitions. The scan sequencer broadcasts the vector parameters defined by the host to the subsystems via scan control bus 18.
The main data path begins with the analog RF inputs to the beamformer 2 from the transducer 20. The beamformer 2 outputs two summed digital baseband I,Q receive beams. The I,Q data is input to a processor 4, where it is processed according to the acquisition mode and output as processed vector (beam) data to the scan converter/display processor 6. The scan converter accepts the processed vector data and outputs the video display signals for the image to a color monitor 22.
The B/M mode processor converts the left/right sets of baseband I and Q data from the beamformer into logcompressed versions of the left/right signal envelopes. The B function images the time-varying amplitude of the envelope of the signal as a grey scale. The envelope of a baseband signal is the magnitude of the vector which I and Q represent.
Ultrasound imaging suffers from the inherent imaging artifact referred to as speckle. Speckle is the mottling found in the images produced from interference patterns of multiple receive echoes. This mottling is primarily caused by the null in the acoustic interference pattern, but other anomalies in the image, e.g., random electronic noise, can cause mottling. The acoustic nulls are accentuated by the log compression required to display the full dynamic range of the ultrasound image. These nulls appear as black holes in the image. It is desirable to minimize speckle to improve image quality.